The present invention relates to a soundproof material, for example, installed on a floor surface within a passenger compartment, in order to improve quietness.
For example, as shown in FIG. 10, a soundproof material disclosed in Japanese Patent No. 3359645 is provided with a soft layer 22 arranged on a vehicle panel 21 serving as a floor surface within the passenger compartment, and a hard layer 23 arranged on the soft layer 22. In accordance with this publication, the soft layer 22 has an air permeability, and an air permeability of the hard layer 23 is set lower than that of the soft layer 22. The mass per area of the hard layer 23 is set within a range between 300 and 2000 g/m2, inclusive. The weight of the hard layer 23 is set greater than the soft layer 22 in order to attenuate a vibration due to sound.
Further, as shown in FIG. 11, a soundproof material disclosed in Japanese Laid-Open Patent Publication No. 2003-19930 is provided with a first sound absorbing layer 24 arranged on a vehicle panel 21, an intermediate layer 25 arranged on the first sound absorbing layer 24, and a second sound absorbing layer 26 arranged on the intermediate layer 25. In accordance with this publication, the mass per area of the first sound absorbing layer 24 is set to 900 g/m2, and the mass per area of the second sound absorbing layer 26 is set to 500 g/m2. Further, the mass per area of the intermediate layer 25 is set within a range between 10 and 200 g/m2, inclusive, and the air permeability thereof is set within a range between 3 and 35 cc/cm2/sec, inclusive.
However, in the case of the soundproof material disclosed in Japanese Patent No. 3359645, in the hard layer 23, it is necessary to make the density high and increase the weight while maintaining the air permeability. Accordingly, it is necessary to use a binder fiber for the hard layer 23, or it is necessary to apply a hot press work. Accordingly, since the weight of the soundproof material itself is increased as well as a manufacturing cost becomes higher, this structure is not suitable for an in-vehicle part. Further, both of the soft layer 22 and the hard layer 23 have the air permeability, and particularly, the hard layer 23 has a comparatively great air permeability (5 cc/cm2/sec or more). Accordingly, since the noise easily transmits through the soundproof material, this structure is not preferable in view of a sound insulating property. In order to cope with this, a resin panel having no air permeability may be used as the hard layer 23. In this case, a manufacturing cost becomes lower, and a sound transmission is suppressed. However, not only the noise within the passenger compartment is reflected by the hard layer 23, but also the noise from an outside of the vehicle is transmitted to an inside of the vehicle via the hard layer 23. Therefore, there is a risk that the quietness within the passenger compartment is deteriorated.
Further, in the soundproof material disclosed in Japanese Laid-Open Patent Publication No. 2003-19930, all of the intermediate layer 25, the first sound absorbing layer 24, and the second sound absorbing layer 26 are formed by the soundproof material. In this case, in order to sufficiently obtain the quietness within the passenger compartment, it is unavoidable that the entirety of the soundproof material becomes thick. Accordingly, it becomes hard to secure an installation space of the soundproof material within the passenger compartment. Further, since the weight of the soundproof material itself is increased, the structure is not preferable for an in-vehicle part. Further, since the soundproof material is constituted by a felt or an unwoven fabric having an excellent sound absorbing performance, there is a defect that a noise attenuation effect cannot be sufficiently obtained, and the noise transmission loss characteristic is low. Because of the above, there is a problem in respect of the matter that the quietness is secured by the soundproof material.